With the increase

of the number of the coating layers (i.e., the thickness of the HfO2 coating), all the modes shift to a shorter wavelength at the very beginning but then continuously move to a longer wavelength (Figure  1c). Figure 1 Fabrication of the microtube and its typical PL spectra. (a) Schematic diagram of the cross-sectional view of the microtube after HfO2 coating (left panel). The inset indicates the multilayer structure of the tube wall. The right panel shows the optical microscope image of a microtube with coating of 150 HfO2 MLs. (b) AFM images of the flat Y2O3/ZrO2 nanomembranes with (left panel) and without (right panel) coating of 150 HfO2 MLs. (c) Typical PL spectra collected from the center spot of the microtube with different HfO2 VX-770 price coatings (0 to 150 MLs with a step of 10 MLs). The marked (asterisk) modes’ azimuthal numbers are m = 70. To make the results more intuitionistic, we extracted the positions of the mode with m = 70 (derived theoretically) and the corresponding first sub-mode and plotted the positions as a function of the number of coating layers, as shown in Figure  2a. 3-MA order One can see that both modes demonstrate the same shift

tendency, indicating that this is not a coincidence. The key factor leading to this bi-directional shift influences not only the circular but also the axial propagations. The phenomenon has not been previously reported in a similar experiment with Al2O3 coating [15], and we will discuss the mechanism in the following paragraphs. Figure 2 Evolution of mode positions and Q -factors with increasing coating layers. (a) Shift of mode (m = 70, main mode Succinyl-CoA and first sub-mode) with increasing HfO2 coating layers. The dark squares and open circles represent the positions of the main mode and the first sub-mode, respectively. (b) Evolution of the

Q-factor of mode (m = 70) with the coating layer. The triangles are the experimental results and the dashed line is the corresponding linear fit. According to the literature, the mode positions show a strong relationship with the evanescent field and the surrounding medium [5, 10], and the interaction of evanescent field with the absorption molecules on the wall of tubular microcavity leads to a detectable shift in the resonant frequency (i.e., mode position) [10, 18] The previous experimental [15] and theoretical [19] results indicated that the resonant wavelength monotonically redshifts with increasing thickness of the high-refractive-index oxide (Al2O3 or HfO2) coating. In the present case, the modes show an obvious redshift with the HfO2 coating increasing from 20 to 150 MLs (Figures  1c and 2a), which fits well with the previous experimental results and theoretical prediction.